The invention relates to transformers and particularly to common mode noise attenuation in transformers in the single phase range of 0.125 KVA to 15 KVA. The invention may also be applied to three phase transformers in ranges up to 45 KVA. The basic transformer has current in the primary that develops a fluctuating magnetic field. The field cuts the turns of the secondary to develop an electromotive force in the secondary. In addition to the desired electromotive force, other components that are not desired also pass over from the primary to the secondary as well as from the secondary to the primary. These undesired components are called noise. For many applications the noise is not objectionable. For many other applications the noise is objectionable and such applications include power supplies for computers and other data processing equipment, medical equipment and other voltage sensitive devices. Problems that may be encountered when such noise is transmitted may include the loss or change of data held in volatile memory or interference with electronic control equipment. For example, noise from a power line may introduce spurious signals into a computer operating system and these signals can be processed as significant data which may result in extra or missing bits which can drastically change the results. Similarly, an important factor is that certain rotating equipment, for example, may impose noise on the power line and this noise may effect other equipment that is connected to that line. Thus it is desirable to minimize both noise transferred from the primary to the secondary of the transformer, as well as from the secondary to the primary. The prior art includes two known methods to achieve high common mode attenuation. The first involves spiral wrapping a coil in a manner similar to a "tire-wrap" using a conductive foil tape. The second uses a shield of relatively thick rigid conductor preformed by a machine into a box-like configuration which slides over the preinsulated coil. Better attenuation is achieved by the first method because the preformed shield is arranged in closer proximity to the coil conductors. The spiral method is, however, undesireable because it is highly labor intensive. The box-like configuration is undesirable because it requires precise dimensioning and tooling, and the shield must be manufactured prior to assembly of the transformer.
The Faraday shield is well known and has been widely used. Applications include the use of a conductive foil placed between coils of the transformer to divert noise to ground. In some cases, capacitance around such a Faraday shield will still couple enough noise from the primary to the secondary to cause problems in very sensitive equipment. It is also known to use variations of the Faraday shield which is essentially a box shield which completely encloses the winding with a conductive foil. The box shield provides a ground path for primary circuit noise and has the advantage that a much smaller capacitance exists between primary and secondary coils than in the case of a simple Faraday shield.
The prior art has used various stamped metallic members which are intended to fit around at least some of the windings of a transformer. Such a prior art approach is shown in the layout view of FIG. 2. It will be seen that known shield involves a relatively complicated pattern which involves difficult assembly onto a transformer winding.
The prior art includes the structures shown in the following U.S. Pats. Nos. 2,978,658 Reaves; 3,983,522 Gearhart; 2,997,647 Gaugler et al; 4,236,133 Seiersen; 3,181,096 Raub; 4,311,977 Owen; 3,717,808 Horna; 4,454,492 Thackray; 3,886,434 Schreiner; 4,554,523 Miki et al; 3,982,814 Kaisrswerth et al; 4,571,570 Wiki et al; 3,278,877 Kameya et al; 3,560,902 Okuyama; 3,678,428 Morris et al; 3,699,488 Goodman et al; 4,042,900 Hinton et al; 4,153,891 McNutt; 4,518,941 Harada.
Of these listed Patents, U.S. Pat. No. 4,042,900 Hinton et al, describes a floating electrostatic shield for disc windings. U.S. Pat. No. 3,699,488 Goodman et al, describes a static shield for each winding section which comprise a strip of aluminum-backed crepe paper. U.S. Pat. No. 4,153,891 McNutt, describes an electrostatic shield assembly for power transformer winding. Similarly, U.S. Pat. No. 4,518,941 Harada, describes two electrostatic shield foils imposed between the primary and secondary windings with an insulator disposed between the electrostatic shield foils. The other patents are only of general interest.
It is an object of the invention to provide effective common-mode noise attenuation without the complexity involved in tooling and stamping a shield such as that prior art shield shown in FIG. 2.
It is an object of the invention to provide apparatus which is inexpensive to manufacture as well as requires a minimum of labor to install.
Still another object of the invention is to provide a shield which is highly effective.
It is an object of the present invention to provide a coil shield which can be fabricated at the assembly floor and does not have to be fabricated by means involving relatively elaborate tooling with precise 4 dimensional controls.